The present invention relates to a method of printing a receiving material including the steps of feeding the receiving material through a printer containing a first and a second image-forming unit, each of the units containing a write head and an image medium. The method includes forming, by means of the first image-forming unit, a first image on the first image medium using the first write head; forming, by means of the second image-forming unit, a second image on the second image medium using the second write head, transferring, in a transfer nip, the first image to the front of the receiving material and the second image to the back of said material. The present invention also relates to a printer suitable for using the present including a method of adjusting the printer.
A method of this kind is known from U.S. Pat. No. 5,970,295. In this method, each of the image-forming units includes a write head for writing an electrostatic latent image on a photoconductive image medium and means for developing said image to form a visible image, using toner. The developed image is then transferred to an intermediate element in the form of an endless rubberized belt. The two intermediate elements of the image-forming units come together at the transfer nip. By feeding a sheet of receiving material through this transfer nip the front and back of the material can be printed substantially simultaneously. This gives the advantage that a sheet does not have to be turned over when it is required to be printed on both sides. As a result, the feeding of the receiving material is simplified and registration errors can be avoided or at least reduced. Minor register errors, however, can occur because the two image-forming units need not be identical. As a result, despite the fact that both images are written at the same time with the write heads, it may happen that one image reaches the transfer nip earlier than the other. A difference in head level, i.e. the position of an imaginary frame around a printed image with respect to the edge of the receiving material situated furthest downstream (also known as the leading edge), between the front and back of the receiving material is the result. A solution to this problem is known from the patent specification. How much time elapses between writing a latent image on the image medium and transferring the image formed therewith to the receiving material is determined by means of reference images for each of the image-forming units. If there is a difference in this time between the first and second image-forming units, then the time at which at least one of the printheads writes is adapted so that this difference is cancelled out. Another possibility is to adapt the speed at which the image media or intermediate elements revolve. Here again the difference in the time can be eliminated.
A disadvantage of the known method is that only registration deviations in the direction of transit of the receiving material can be remedied therewith. Adjustment of the writing time or a change of the speeds of revolution of the image-carrying media only gives possibilities for adjusting the position of an image on the receiving material in the direction of transit of the material. Although the registration errors that can be expected are probably more pronounced in this direction, the increasing demands made by users with respect to register quality means that even minor registration errors in a direction extending transversely to the direction of transit of the receiving material are found to be extremely disturbing.
For printers which use only one image-forming unit various solutions are known from the prior art to prevent registration errors in the lateral direction.
One solution proposes determining where the image is situated in the transfer nip, the sheet of receiving material being fed to the transfer nip in such a manner that it exactly coincides with the image. In this method, abbreviated as “image-sends-sheet,” the sheet of receiving material is thus, in each case, sent to the corresponding image depending on where the image is situated in the transfer nip. In a printer with two image-forming units, use of this method results in good register of only one of the two images. A choice must then be made as to which of the two images is used to send the sheet. The position of the other image on the sheet of receiving material is then an uncertain outcome.
Another solution proposes measuring where the sheet of material is situated and so writing the image in dependence on the measurement in the lateral direction that the image coincides with the receiving material in the transfer nip (“sheet-sends-image”). This solution can in principle be successfully used in a printer with two image-forming units. However, since the instant of writing takes place long before the sheet of receiving material really is present in the transfer nip (the image must of course first be formed on the photoconductor, then transferred to an intermediate element and then transported to the transfer nip), the position of the receiving material in the transfer nip cannot yet be established with high accuracy at that specific writing instant. Consequently, it is practically impossible in this way to obtain very good register accuracy transversely of the direction of transit. Another disadvantage of this method is that deviations in the sheet transport may be relatively considerable, up to 10 mm deviation from the nominal (required) position. If such deviations have to be absorbed by adapting the image formation to these deviations, considerable tolerances are required for the image-forming units. This makes these units expensive and very bulky.